This invention relates generally to densitometers for measuring optical density. In particular, the invention relates to optical density measurement of toner-covered test patches or other areas for controlling process parameters in electrostatographic apparatus such as copiers and printers.
In electrostatographic apparatus such as copiers and printers, automatic adjustment of process control parameters is used to produce images having well regulated darkness or optical density. Copier and printer process control strategies typically involve measuring the transmissive or reflective optical density of a toner image on an exposed and developed area (called a xe2x80x9ctest patchxe2x80x9d) of an image receiver. Optical density has the advantage, compared to transmittance or reflectance measures, of matching more closely to human visual perception. A further advantage, especially for transmission density, is that density is approximately proportional to the thickness of the marking material layer, over a substantial range.
Typically, toned process control test patches are formed on the photoconductor in interframe regions of the photoconductor, i.e., between image frame areas. An xe2x80x9con-boardxe2x80x9d densitometer measures the test patch density, either on the photoconductor or after transfer of the patches to another support member. From these measurements, the machine microprocessor can determine adjustments to the known operating process control parameters such as primary charger setpoint, exposure setpoint, toner concentration, and development bias.
A transmission type of densitometer is particularly well suited to transmissive supports. In this type, a light source projects light, visible or infrared, through an object onto a photodetector such as a photodiode. In a copier/printer, the photoconductor passes between the light source and the photodetector. When the photoconductor has toner on the surface, the amount of light reaching the photodetector is decreased, causing the output of the densitometer to change. Based on this output, the amount of toner applied to the photoconductor can be varied as required in order to obtain consistent image quality. Another type of densitometer as described in U.S. Pat. No. 4,553,033 to Hubble, III et al uses reflected light flux rather than transmitted light flux to determine density, and is particularly suited to opaque reflective supports.
Whether of the transmissive or reflective type, the densitometer photodetector signal is input to signal processing circuitry, either analog or digital. The modem trend is toward digital circuitry, such as disclosed in U.S. Pat. No. 5,117,119 to Schubert et al. The Schubert et al densitometer is auto-ranging, where one of several available ranges is utilized according to the density of the test sample. The individual ranges span a factor of 10 in transmittance or reflectance, equivalent to 1.0 density units.
Photoconductors tend to be fatigued, i.e., degraded in their photoconductive characteristics for subsequent imaging cycles, after long or repeated exposure to light, resulting in degraded image quality. Improvements in photoconductor formulation, disclosed for example in U.S. Pat. No. 4,397,932 to Young, have been helpful in reducing the fatigue problem, but fatigue remains an issue for many photoconductors in use today. For this reason, copiers and printers are typically designed to minimize the exposure of the photoconductive member to unwanted non-imaging light, such as room light. Light-tight machine enclosures, careful service procedures, and careful photoconductor belt or drum packaging and installation are typically used to minimize the room light exposure. However, light sources within the machine enclosure remain a potential cause of photoconductor fatigue.
In some instances, light sources within the machine enclosure can be shielded from the photoconductor. This can be effective with electro-optical devices commonly used to sense the position and motion of image receiver sheets, for example.
In many copier/printer configurations, an on-board densitometer employs a light source directed at a spot on the photoconductor, for the aforementioned purpose of process control. Shielding would defeat the function and purpose of the densitometer. After prolonged exposure of the photoconductor by the densitometer light emitter, fatigue and image defects can result. The severity of the problem depends on the spectral sensitivity of the photoconductor and the spectral emission of the densitometer light source, as well as the intensity and duration of the exposure.
In some applications the spectral sensitivity of the photoconductor does not match the spectral emission of the densitometer light source. For example, a visible red-sensitive photoconductor may not be significantly affected by the emissions of a densitometer having an infrared light-emitting diode (LED) light source. In that case the emitter may be left fully energized indefinitely, even when the photoconductor is motionless between print jobs, without causing image defects. Continuous operation in this manner is not only convenient, but also avoids warm-up effects except when first turned on.
However, many preferred photoconductors have spectral sensitivity extending through the visible range into the near infrared. In these cases the infrared light emitted from the densitometer and incident on the photoconductor can cause significant fatigue, leading to defective images. The problem is most acute in any spot on the photoconductor that is parked motionless opposite the energized densitometer light emitter when the machine is idle between jobs.
Another problem with on-board densitometers is that their typical LED""s or other emitter types do not have the ideal constant light intensity for density measurement. Short-term instability results from temperature sensitivity during warm-up. In continuous-mode operation at a fairly high LED current, unstable warm-up periods of a minute or more are commonly observed. Additional longer-term instability results from gradual degradation of the LED with age. Complicated and expensive approaches may be required to avoid inaccurate measurements due to these instabilities. Such approaches include extended warm-up periods, temperature compensation, intensity feedback control, and periodic recalibration.
Operating an LED in a pulsed-mode is a well-known approach to reducing average power dissipation and reducing PN junction temperature rise. When a current pulse is applied, the typical LED PN junction temperature response includes a component with a fast time constant of about 10 to 40 milliseconds. Pulsed-mode operation with a pulse-width much less than this, say a few hundred microseconds or less, along with a low duty-cycle, minimizes temperature rise, improves light emission stability during warm-up, and prolongs LED useful life.
In densitometer applications, pulsed-mode operation has been used to isolate a density signal from an ambient light or noise signal, as in U.S. Pat. No. 5,173,750 to Laukaitis, and U.S. Pat. No. 5,900,960 to Reime. Benwood et al (U.S. Pat. No. 3,830,401) use pulsed-mode LED operation to monitor the reflectivity of a developer mixture of relatively reflective carrier particles and light-absorbing toner particles. Butler (U.S. Pat. No. 5,119,132) discloses a pulsed-mode reflection densitometer suitable for both black and colored toner over a wide density range. Pulsed-mode operation has also been used to enable higher intensity for measurement of higher-density samples, as in U.S. Pat. No. 4,068,956 to Taboada. For densitometry of toner images on a photoconductor, pulsed-mode operation obviously reduces the exposure of the photoconductor, compared to continuous operation at the same intensity. The reduced exposure of pulsed-mode operation can be beneficial in reducing the aforementioned problem of photoconductor fatigue.
In the case of a moving photoconductor, a problem with pulsed-mode densitometer operation is that too low a pulse frequency can yield measurements too widely separated on the test sample. On the other hand, too high a pulse frequency results in greatly overlapping measurement spots on the photoconductor. With a fixed minimum pulse width required for measurement acquisition, a pulse frequency too high, with a high duty-cycle, sacrifices much of the exposure reduction advantage of pulsed-mode operation. With the greatly overlapped measurement spots, there is little compensating advantage gained in measurement continuity.
Another difficulty with densitometer pulsed-mode operation is that the photodetector output is valid only when the LED is energized, and these times might not match the times when density readings are needed on the moving photoconductor. That is, the density readings might not be taken at the spots where readings are needed, such as on process control patches. Another potential problem is that the time when the LED is pulsed might not match the time when the host processor or LCU reads the densitometer output.
While pulsed-mode operation reduces exposure, it does not totally eliminate the problem of fatiguing the photoconductor in spots parked motionless opposite a pulsing LED. Time periods of minutes or even hours between jobs are quite common, even in high volume production environments. Depending on the photoconductor formulation, even a low duty-cycle pulsed-mode exposure of such duration could severely fatigue any spot parked opposite the pulsing LED. This could result in persistent image defects in subsequent jobs, or require costly replacement of the photoconductor.
Another problem with on-board densitometers is related to initial setup and service. For test, calibration, and diagnostic purposes, a service technician will typically carry one or more calibrated density test standards for insertion into the on-board densitometer. The densitometer output is then monitored to verify densitometer function and accuracy. Gaining access to the densitometer for these tests is inconvenient, requiring the opening of machine covers and, in many cases, removal of machine parts. Moreover, the tests cannot be performed if the test standards are unavailable.
One object of the present densitometer invention is to stabilize LED intensity and prolong the LED useful life by operating in a pulsed-mode. Average power and temperature of the LED is greatly reduced. This permits operation at higher intensities for brief pulses, enabling density measurements of higher-density test samples. The lower temperature rise results in more stable light output, and prolongs the useful life of the LED. The pulse frequency is selected according to the effective measurement spot size and photoconductor velocity, so as not to degrade densitometer data collection capability in a copier/printer application. Further stabilization of LED intensity is obtained by monitoring a signal representing the densitometer emitter output and feeding back to adjust the emitter drive circuit. The emitter drive circuit adjusts LED current in a manner to regulate the LED light output.
Another object of the present invention is to hold the density output signal steady between LED pulses. This prevents invalid density readings from being presented, either to a human-readable display or to a host processor.
Another object of the present invention is to reduce photoconductor fatigue caused by exposure from the densitometer light emitter. The densitometer pulse duty-cycle is kept low, so that total exposure of the photoconductor is greatly reduced, compared to continuous mode operation. Again, the pulse frequency is selected according to measurement spot size and photoconductor velocity, so as not to degrade densitometer data collection capability in a copier/printer application.
Still another object of the present invention to prolong LED useful life and reduce photoconductor fatigue by automatic reduction of exposure from the LED between jobs. The LED drive circuit is connected to a photoconductor motion status sensor, such as an encoder. If the sensor indicates no motion, an interval timer begins and times out if the photoconductor remains motionless longer than the timer setting. When the timer times out, the LED is switched to a low intensity or off, or the pulse duty-cycle is reduced substantially below the normal duty-cycle value.
Full intensity and normal duty-cycle are restored when motion resumes. If the normal pulse duty-cycle is low enough, the low temperature rise largely eliminates the warm-up problem that occurs with continuous mode operation. An override may be provided to restore normal duty-cycle and intensity while the photoconductor is motionless, permitting density measurement of a motionless photoconductor.
It is yet another object of the present invention to provide a functional test and calibration of the densitometer. On a command signal, the densitometer LED drive circuit reduces the LED intensity by a calibrated amount, simulating the insertion of a calibrated test standard. There is no need for the service technician to carry calibrated test standards and no need to gain physical access to the densitometer. Time required for densitometer test and calibration is reduced, and there is no risk of lost, forgotten, smudged, or damaged calibration standards.
To obtain these objects, a densitometer with a controllable light emitter intensity is disclosed. During normal printer operation, the emitter may be operated in a pulsed-mode, thereby reducing emitter temperature and prolonging its useful life. Exposure of the photoconductor is also reduced, which may prolong its useful life and avoid image defects. The pulse width, frequency, and duty-cycle are chosen to give these benefits without compromising density measurement capability.
Whether in pulsed or continuous operation, the emitter drive circuitry is connected through a timer to a motion status sensor so that exposure from the emitter is reduced below the normal level when the photoconductor motion is stopped. A test mode is provided during which the emitter intensity is reduced by a calibrated amount, simulating the presence of a calibrated test standard. An emitter stabilization circuit regulates the emitter light output, even when there is no room to position a sensor to monitor the output of the emitter shining on the photoconductor.